Portal effect with two different hdri's
Use a light path node and color mix node.

On the so called "portal" object use a transparent shader.

Then on the world, set the two different HDRs and use the transparent depth to control the RGB mix of the environments. 

Don't use shader mix nodes to mix RGB color information. Pay attention to the color of the sockets! Green sockets should connect with green, yellow with yellow, gay with gray and so on.

Also: nodes that are not connected to the final output have no effect on the result.

Read:
What is the meaning of the color of the node sockets in the node editor?

and 

Cycles - (Shader) Nodes - inputs outputs - What are the exact data types?


How this all works is

Transparent rays will use one image, all other rays (like diffuse or glossy, etc) will use the other. The limit of this solution is that glass objects (or other transparent or semitransparent objects) will need to be composited.